It's likely fixable. It might need some switches replaced, or there might be some damaged circuit board connections or traces that could be re-soldered or bypassed. I think any fix is going to require soldering, and maybe a multimeter. Whether fixing it is cheaper than buying a new board depends on whether you can borrow tools, and the cost of replacement switches if you need those.
I know the switches are not hot-swappable, but you can de-solder switches on just about any mechanical keyboard. Add a solder sucker to your tools list if you need to do that. There are guides online for replacing keys on the specific board you have.
More details would be helpful for diagnosing the problem:
- Are there rows or columns of keys that don't work? (This could indicate a problem with the circuit board, or maybe a diode that needs to be re-soldered or replaced. I don't know if the Blackwidow has diodes or not. In any case it's multimeter time.)
- Or is it a key here and there that doesn't work? (This is more likely to be a problem with switches. The might need their solder joints touched up, or they might need to be replaced.)