My code development was progressing until now, then the modem suddenly started returning ERROR for every AT command I enter! AT // Returns OK All other commands returns ERROR. What could I have done to cause this to happen?

Do you remember using the +WPIN commands?
This command enables the lock code. If this lock code is enabled, the modem will not process any commands unless the code was entered.

Security PIN +WPIN Description:
This command sets, enables, or disables the security PIN. When this PIN is enabled, only ATD (emergency numbers only), ATH, and +WPIN commands will be accepted.

Note 1: The default value of the WPIN will be the last four digits of the mobile directory number if configured (+WMDN). If not configured, the default value of the WPIN will be 0000.
Note 2: When the security PIN is enabled, the unit will power-up in the "locked" state.
Note 3: For RUIM software builds, the default value for WPIN is '0000' even if a WMDN is present.

0 Disable modem lock
1 Enable modem lock
2 Change the PIN number
3 Verify the PIN. Unlock the modem until reset. Does not change NV enable/disable setting.
4 Enable modem lock upon power up

Try disabling the Security PIN by sending this command: 'AT+WPIN=0,ssss' (where ssss = the last four digits of your MDN).