[inquisitor]

Welcome. Since MTel's website is not clear in this regard, I wonder if roaming is completely disabled for those unregistered SIMs or if you can register on foreign networks and use at least USSD. The latter would allow you to redeem a top up voucher bought before leaving the country from abroad in order to extend SIM validity.
Your SIM should be valid for 12 months after activation or your last recharge.

Btw postpaid subscriber of Mtel can recharge prepaid SIMs by SMS with 6, 10 or 15 Leva, which will be charged on the next monthly bill. So if you happen to know a postpaid subscriber on Mtel ask him to send your number followed by a comma and the desired amount to 1616. E.g. to send 6 Leva to a prepaid SIM with the number 08812345678 the postpaid subscriber would need to send the following text:
08812345678,6

In case you need to keep your SIM alive, I can probably arrange such a transfer for you.