If you put only one question mark after a sentence, your message will be posted on the forum without delay.
Writing a thesis! Are you a future PhD? In what?
Anyway, I trust that you are making backups of all your documents.
The issue you are experiencing may have several causes.
You may not have enough ram on your computer to open several documents. Thesis documents, I know, are large.
Your document file may be to large in volume and may need to be separated into smaller files.
You may have integrated text or images in another format other than word.
I recommend that after each work session, back-up your files onto another memory device. Then, delete all of your temporary files using CCleaner which you can get here and it's free: http://ccm.net/download/download-33-ccleaner
Let me know if this solves your issue.