Windows form crashing to taskbar

Hi, i am writing whats turning into quite a large (little) program that displays thumbnails (1 - 200) in a a form which also has a timer which activates a backgroundworker process which updates a listbox with info.

It all works fine untill i leave it idol for an hour or so then the form is only visible in the taskbar (its icon) . Its as if its minimized but clicking it activates (the taskbar icon changes colour etc with focus) but it wont maximise.

If i use the taskmanager it says the program is 'Running' and by right clicking and selecting 'Maximise' the window will appear maximised but all the controls on the form arent displaying the images or listboxes, its as if the application has crashed. I need to 'end task' to exit the program the menus and close (x) icon dont respond.

Like i said the application runs fine if i interact with it, it appears to only 'crash to taskbar' if i leave it unattended for an hour or so.

Any ideas where i might start looking to resolve this?

Thanks