I think your problem is caused by hidden temporary files locking your document
When you open a document, a hidden temporary file is created in the same folder. It will be named similar to the document. For example, when you open ‘myfile.ods’, a temporary file named ‘.~lockmyfile.ods#’ will be created.
Usually, they are removed when LibreOffice closes but if it closes unexpectedly, the temporary file may persist and prevent you from opening the document.
To view hidden files: I assume you have moved TO Windows 7 from XP.
Open a Windows Explorer by holding down the windows key and pressing ‘e’ (WIN + E), or Start - All Programs - Accessories - Windows Explorer.
Organise → Folder and Search Options.
View Tab: select ‘Show hidden files, folders, and drives’.
You should now be able to see any hidden LibreOffice temporary files in the same folder as your documents. Delete them and your documents should open.
I hope this helps. If not, please provide more details. For example, the exact wording of any error messages you may be receiving.