There are a couple of things to check.
Make sure that if you have a battery cutoff switch that it is turned on.
Make sure the cables to the battery are of correct polarity and tight.
Turn off then on the breaker for the converter.
Look behind the wall in the basement and make sure that the converter is plugged into the 120 volt receptacle.
There are also two or three fuses on the converter itself. Check to be sure that none are blown. If the previous owner installed the cables to the battery wrong, that will blow those fuses.
If all of those check out okay, use your meter to see if there is 13+ volts DC at the converter output.
The converter itself may have died.
Let us know what you find.
There may be more suggestions coming from others.
Peace
Dave