You had dormant demons in the tank, you woke them up
Seriously, you may need to drain and refill the tank.
If you are lazy ( like me ), you could disconnect the flexible line where it connects to the manifold and use a rubber tube connected to it to dump the fuel into 5 gallon cans.
Hook up a battery charger and turn the key on ( not start ) to energize the fuel pump.

( ALL THE ABOVE IS ASSUMING THAT THE FUEL IS THE PROBLEM, NOT SAYING IT IS )