Linda,
Unthread it. Pull out the needle. Pull out the bobbin and double check the hook and the area around it.
If you have the manual reread it. Do you have the needle in correctly? Is it the right needle? Is it threaded correctly? Is the bobbin case in properly? And like that.
I'm not familiar with a Singer Lightweight but it's a sewing machine so basics apply.
Hope you get it figured out quickly.
Joe