A family of Microsoft word processing software products for creating web, email, and print documents.
First, if you're having trouble using the mouse to select text, try using the keyboard: hold down the Shift key while you use the arrow keys to move the cursor to the end of the text you want to select. Does that work?
If neither of those things work, try it in a different document. If it works there, it could be that the first document is protected. Reopen that document, go to the Review tab, and click the Restrict Editing button at the right end of the ribbon. In the panel that opens, is there a Stop Protection button at the bottom? If so, click it.