It sounds like a problem with your web server, assuming that the email address you're sending to is correct.
Upload the following script to your web server,*put your email address in where indicated,*and open it with a browser.
If you get the error message, then contact your hosting provider and ask them why this doesn't work.
If it works, then we need to investigate whether you've made an error in your form or in configuring FormMail.
Here's the script, and it's also attached:
if (mail("YOUR-EMAIL-ADDRESS","Testing","This is a test"))
echo "Mail sent";
echo "Error sending mail";