iTunes can't see my songs. Drive Letter Change.

I have over 10,000 songs in my iTunes library. They all have a path that originates from my drive L. This is my external hard drive. The drive was shut off, as it has been many times in the past, and when it was turned back on, it was all of a sudden drive M.

Now iTunes cannot find anything in my library. I tried the Disk Manager in Win7 to reassign the drives letter back to L but it won't let me do it because the final step is to pick the drive's new letter assignment from a drop down menu, and "L" is not available.

Can anyone help me with this. Is my iTunes simply just useless from here on out. I'm not deleteing it and starting over again just so the drive letters can switch again.

thank you for any help.