What a disappointment! 250 is a lot of tulips. What comes to mind are 2 things:
1. How deep did you plant them? I find deeper is better. 9 to 12 inches deep give mine better chance of survival in subsequent years.
2. What type of tulips did you plant? Some unfortunately are basically annuals. I've had species and Darwins come back for years and years.
Oh and one more thing - are they in a well-drained spot? Could they have rotted from too much moisture?

I thought the same as Jerseygirl. Tulips, as other spring bulbs, need dry conditions in the summer while they're dormant. If you dig around where they were you may discover what happened.

Darwin hybrids are generally the most perennial of tulips, but the sad reality is that tulips in general are not long lived. Some will return for a few years, but most will not.

if treated like bearded iris - that is, kept dry and baked over summer, they are more inclined to return. The shaded soil (zinnias) may not have helped....but I would still expect to see evidence of growth, even if the plants were either blind or small flowered. I would suggest having an exploratory dig about.