The lasso tool says I don't have any pixels and won't let me copy

I’m having increasing troubles using the lasso tool.  At first, I could use the lasso tool, then copy/paste over a spot I wanted to cover up. It worked except I had to do Paste 7-8 times to cover up the bad spot. Now, I can’t even lasso and do “copy” because it says I don’t have any pixels selected. I have restarted my computer, and that doesn’t help.  What does that mean? How do I have pixels selected when I lasso/cc?  Why did it used to work and now it doesn’t?