Each new version of b2evo brings new features and changes, every now and then this will break skins.
Sometimes you will get visible errors, sometimes you will end up with invalid xhtml, sometimes things won't work right, and sometimes you won't notice anything at all.
What would be the best and first thing to do would be to check the skins page to see if the skin you want is already updated, or maybe find a skin you like better, you can do that here:
This page in the wiki (yes we have a wiki) will help you convert your skin to work in a newer version, and will list which versions will break your skin:
If you are just a average user and know nothing about skinning, you can make a post in the appropriate topic to request for someone to update your skin:
[url=http://forums.b2evolution.net//viewtopic.php?t=10580]1.9.x Skin Upgrade Requests[/url]
[url=http://forums.b2evolution.net//viewtopic.php?t=8980]1.8.x Skin Upgrade Requests[/url] - Skin Up/Downgrades no longer supported
Note: Skin Up/Downgrades are only supported for the latest versions of b2evolution.