With the PC powered off, remove the CMOS backup battery (CR2032) for AT LEAST 10 minutes (Intel says 15 min).  Reinstall and then try.  If it still will not boot I would suspect the BIOS was somehow corrupted.   There are many Inspiron 15 models.  What is the exact model number? 
I tried installing Ubuntu on my Inspiron 15, but now on startup, it gets stuck on the Dell loading screen. None of the buttons work, f2, f12, all that stuff doesn't do anything (pic included). I tried doing the hard reset by taking the battery out and holding the power button, but that didn't do anything. Anyone got any ideas?[IMAGE]
Laptop stuck on Dell screen on startup