Why do I get a NO VGA SIGNAL on my pc?

Hello,
I am really worried that I may have to buy something and I really dont have the money to but my pc keeps stating there is no VGA signal..its a 2yr old gateway GT5428...I dont know what to do and it beeps 3 times everytime I start it up...It never did this before and I dont know how to check for the memory like the internet search keeps saying is there anyone out there that has had this problem?? Please help all answers are appreciated.