300 and 505's are next to impossible to brick (this would be the first one) at least with PRS+ installers (which were used by thousands of people).
Now, there are 2 possible reasons of your problem:
1) internal memory stuff got corrupted book and that's why it hangs during boot. If that is the case, it should still boot if you reset it (1. On the bottom edge of the Reader, using a pin or paper clip, press the RESET button. 2. Turn on the Reader.) and immediately connect to your PC. Then removing all content from internal mem should fix it.
2) Fsk flashing failed. In this case, you will have to reset it 10 times in a row (right ater yellow lamp blinks after previous reset) and it will fall into firmware update mode. In that mode use
this updater (it's a modified 505 version, you can't keep it) then repeat PRS+ installation.
If then you are unhappy with PRS+, post about it here and I'll create an "uninstaller" for PRS 300 (which is long due since many people are afraid of trying PRS+ since there is no obvious way back).