Inheriting an alarm system in our first house
Speaking in general terms, the wired sensors are standard across manufacturers. If you call an alarm company to come out and hook you up for monitoring, they will either reuse the panel or just replace it. Often this cost is waived as part of signing a monitoring contract. Wireless sensors are not as compatible, but not out of the realm of possibility.

Monitoring does not require a land line. Cellular monitoring is available.

If you can find the model number of the main control board, often the programmer or installer manual is easily locatable on the internet. These manuals can have instructions on resetting passwords.

If you can't find the manual or you can't reset it, and you're feeling a bit more DIY, you can buy a new control board and install it yourself.