I took my iPhone 4 apart and now the display isn't working
I concur with Jan's suggestion, and direct you to iFixit's Installing iPhone 4 Display Assembly. Perhaps if you repeat the procedure, carefully following the instructions in the guide, you can restore the display functionality.

I would also add, at the risk of sounding condescending, that had you done a little research prior to tearing open the device, you would have discovered that the iPhone 4 digitizer is bonded to the outer glass. This was much discussed, and often criticized both because of screen discoloration on new devices (until the "glue" set) and as being anti-DIY repair when the device was first released. Live and learn, I applaud your initiative in opening your device.