I wouldn't waste time and disrupt everything you're doing by restarting your computer; it's standard advice used by tech people to create a known starting point, but rarely has anything to do with the problem. As Janet explained, there are issues with tokens expiring, which results in messages telling you that you can't do a perfectly valid action. Restarting the computer doesn't fix that, and guarantees that you lose your work unless you have saved to a file outside of the web browser.
The solutions to timing out if taking a long time to prepare a post are generally to
- "Preview" occasionally to keep the session active, and to
- select all the text in the compose box of the post (click anywhere in there and Ctrl-A in Windows) and copy it (Ctrl-C in Windows) before hitting "Submit"... in case you need to start again you can just paste (Ctrl-V in Windows) what you copied into a new compose window.
If you do the "select all" thing click somewhere to cancel the selection after you have copied it; if you don't, the next thing you type replaces all that stuff you saved, and you need to remember to "undo" (Ctrl-Z in Windows) before it is lost!
If you have copied a draft version of your post (Ctrl-C) then you can paste it into a text file on your computer in case things go really wrong before you're ready to submit, or in case you just want to finish it another time.