I have the original Milestone 2.1 update on my phone. I rooted it with Universal androot. After some days I downloaded Eris market fix. And then I had to restart, but after the Motorola logo nothing happened.
I did the exact same mistake (market fix)... after first reboot device did not boot anywhere after the M -logo. I used that as a good sign to get into cutom ROM's and with some nice quides here and little bit googling (to find the right bootloader) my device is now amazing... I love the G.O.T -menu and Nandroid is a must... even for backin up before doing any stupid moves, like market fix...