iphone 4s says wifi unavailable

I have recently got my first iphone. Its second hand and came from a respectable seller

It worked extremely well for the first while, but today has frozen twice, the first time I had to wait for the battery to drain before I could do anything with it.

It now says the wifi is unavailable?!

Does anyone have any ideas of hints/types on why it is suddenly saying this?

Thanks in advance. Any advice would be much appreciated.