I would suspect you dug them a little early then left them on top of your washing machine too long and at too high a temperature.

Immediately after the first frost and after leaves have blackened, cut the stem back to about six inches above the ground and dig them up. Remove surplus soil from the tubers.

Place the tubers upside down in a dry airy space just long enough for any moisture to drain out of the remaining length of stems. The tubers need to be completely dry before they are stored for the winter - but check on them constantly while drying - it doesn't take all that long and when dry they should still be firm like potatoes. Next store the tubers in trays of dry sand or peat moss in a cool, dry cellar or storage area at about 40 to 45 degrees F. Never store at a much higher temperature, as dahlia tubers will shrivel rapidly.

Once in storage, check on them periodically and discard any that might show mold, etc. - it happens even when everything is done right.

Lots of good websites with pictures available by Googling storing dahlia tubers.

You let them dry too much :-) They can't be allowed to dry out and tubers that are allowed to dry to look like "raisins" are much too dry to remain viable.

IMO, these are some of the easiest of the summer flowering, tender bulbs to store. You can let them go through a light frost to kill back the tops or cut them back to about 4" when the foliage starts to go over at the end of season. Dig them out carefully and wash the soil off. Let them air dry a bit to remove excess water then wrap them individually in newspaper or store in a cardboard box filled with slightly moistened sawdust or peat moss. Keep each tuber from touching its neighbor - this can create rot. Store in a cool, dry place over winter.

Check periodically to make sure the tubers are not too dry and spritz with water lightly if necessary. And make sure there is some air available - don't store in a plastic bag, etc.

If you grow them in containers, you can follow the first part of these instructions but rather than digging, allow them to remain in the container over winter under similar conditions. Let the container soil dry out naturally to a barely moist state and make sure it stays just barely moist and doesn't dry completely. If they are stored too warm, the plants will want to begin to grow again so keep at temps right around 40F, not much more.

I wait until spring planting time to divide. The 'eyes' or growth buds are more visible then and each division must have at least one of these eyes.

Thank you! I thought it might be too early, but it was the end of October. They still had plenty of flowers on them when I cut them. I was afraid of the ground freezing. Maybe I was too early.

The problem is, I don't have a cellar or a garage (I'm in a townhouse) and funny but the basement is the warmest part of the house in the winter. I really don't know where it would be cool enough in my house for storage. Outside it's out of the question, we get down to minus 20C on a regular basis.

I do the google homework, thank you!

I was typing as you ( Gardengal) sent me an answer, thank you to you too. I'm so glad to read your answer ( that they're easy tubers) I didn't know how long to dry, and they kept drying until dead. I'll try dahlias again, I love them too much not to try!
Thank you!

Gardengal has the right idea. Cool storage really is ideal. Too bad so many houses don't have any cold rooms any more.

Here is a link that might be useful: How to overwinter summer bulbs

How about storing them in a box in the fridge, or in a ziplock in the crisper? If you are looking at less then say, 20, they really don't take "that" much room.

I pulled mine out of the ground before the ground froze, and stored them. What I do is take them up, clean them off running under water, soak for around 15 min in a mild bleach solution, then in No Damp (an antifungal agent) for a few more hours. Take them up, and leave them in a dish rack in the sink for around 12 hours to dry. Then I place them in wood shavings, and packed them away into the fridge! Worked like a charm, and these where difficult tubers as my MIL had let them dry way way too much! And, my own tubers where just not very mature so they where so so small with thin necks! Now I decided to pull them out of storage early, and have 30 plants growing in various windows around the house!!

It seems to take about two growing seasons for those cell-grown dahlias to form a 'hand' of tubers. It may be better to keep them in pots for their first winter - if you have the room.

If you grow tuberous begonias - or potatoes - they're quite similar.

Just remember to label them for colour so you don't have to wait for weeks to make your garden pictures and check them frequently in early spring to catch the early sprouters for potting up and placing into more light.