rootinghaven is correct. You will have to make a new apn to get service back. You should be able to do this with any custom roms that don't give you cell service.
Go to: settings, more.., mobile networks, and or access point name. Here you will make a new apn by clicking the + button.
Here you will need to fill out the proper APN settings and save the new APN (anything left out from this list doesn't need to be changed). The settings are:
Name:
MetroPCS
APN:
fast.metropcs.com
Proxy:
Not set
Port:
Not set
User Name:
Not set
Password:
Not set
Server:
Not set
MMSC:
http://metropcs.mmsmvno.com/mms/wapenc
MMS Proxy:
Not set
MMS Port:
Not set
MCC:
310
MNC:
260
Authentication type:
None
APN type:
default,mms,supl
APN protocol:
IPv4
APN roaming protocol:
IPv4
Enable/disable APN:
APN enabled
Good luck!!