GDI error and locked out

Having upgraded from1.40 to 1.60, I got a GDI error.  All the pages that do not have images seem to work ok.  Any page with an image crashes.

I would post more details but I have closed the store and I can't get back in.  Is there a url that needs to be used so that I can login and get to the backend?

Give required write permissions as described here

Thanks for the GDI answer.  

I know the answer to this next bit is going to be obvious, but I can't find the answer in the user guide... Now that I have closed the shop, how do I open it again?
In other words, how do I logon as administrator in order to un-tick the Close Shop box?

jwallis wrote:
Go to your admin area (http://www.yourStore.com/administration/). Then 'Configuration' > 'Global Settings' and ensure that 'Store closed' is checked

Thanks.