Sim not installed??

Hi all

Lately when I look at my phone it will say no sim in the top left and then when I unlock it I get a message saying no sim installed? When I reboot it usually fixes the problem but last night it did not fix it. I was then forced to eject the sim and reseat it which then made it work again.

Any idea what is doing this? When it says no sim I can see all of my contacts so I'm pretty sure it's reading the sim, unless the contacts are stored internally. Any help would be great. Thanks a lot.