what user were you previously using? it wasnt 'root', was it?
using a console/terminal, 'cd' (change directory) to the subdirectory where your files your trying to access are. For example, say you have mounted your windows partition into the folder /mnt/windows.. then 'cd' into /mnt. from here, do a 'ls' (list) command on that folder to view its permissions to see if the user you are SHOULD have rights to it.
check out this
link to learn about file permissions. after reading that you should be able to determine if your current user should have rights to this folder.
if theres still a problem or you need more help let us know