Unable to log into admin

It would appear that the blank screen when trying to log into admin is a common complaint…and from what I can see…no answer has been put forward.

Here is my tail of woe.

I bought a handful of themes sometime ago to create a community web site….classifieds….local events and business directory.

Having created half of the site I had to stop work due to other commitments but all themes were up and working as per.

I have now started to work again only to find that I can not log into the admin panel of any of the themes???????….

[If I install a fresh version of WordPress it works fine with the default themes.]

The login page is there but once filling in the appropriate details all that I get is a completely blank page.

One of the themes came from a different supplier so it does not appear to be the fault of the themes..I think.

I have tried an older version of WordPress….no luck!

I have downloaded and installed a free theme which works perfectly.

I have manually installed the latest version of WordPress…

I have disabled all plugins.

I have done just about everything I can think of including standing on one leg while waving my wifes knickers over my head and whistling Ave bloody Maria!!!!!!

Any thoughts?, chaps.