not start the following day

I just replaced the battery in my 1993 940 wagon only to have it not start the following day. What could cause this to happen? I had a problem with my fuel pump which I replaced. Drove the car for about a month and one morning the won't start. The engine would barely crank over, thought it was a bad battery, charged it and it started. Drove the car where I needed to go and when I got back into the car it won't start. Thinking the battery was dead, I replaced the battery that afternoon. Got up the next day and the car won't start.