My computer is acting as though Thunderbird is not installed.

I recently started using Thunderbird for my verizon/aol account. Aol helped me and there were many problems. I thought everything was OK. When I logged on this morning, my messages were there. After going through today's messages, I left Thunderbird. When I tried to send a message there are problems. First, no new messages downloaded, which is unusual. Second, when I clicked on 'new message' nothing happened. When I click on the TB icon on my desktop, it wants to start off at the beginning downloading and installing. Since I've already had a hard time with this, I will not attempt it by myself. Yesterday I was able to type out an email, but TB would not send it. After aol worked with it, I could send out an email. I think some of the problem is related to the fact that my email account is @verizon, but it is now serviced through aol.