Locked up engine on riding mower?

I have a Husqvarna riding mower....maybe 3 or 4 years old.
About 2months ago, I let the oil get low before I checked it. It did not have much at all. Added oil and it ran fine for like a month. Then I accidentally ran over a tee shirt, which of course, wrapped tightly around the top of the blades and locked them. It took a while, but I was able to cut out the shreds.
Again, the mower ran fine for the next few weeks.
This past weekend, I used the mower, (ran fine again), and parked it when I was finished. It sat maybe 30 minutes, then I was going to move the mower. It clicked, but would not turn over. After inspection and removing the motor cover, we found that we were unable to turn the motor....it was stuck.
My husband says we should "junk" the mower, because I let the oil get too low, but it ran fine for like 2 months afterwards, so I can't believe that is the problem. Please help?
Thank you!

Ann
April 2012