I was trying to sync my libary in a new computer but i mystakenly replaed my online library with my local one (which is empty in my new computer). Can I undo this?
If you were syncing multiple computers before, you can likely restore from one of the automatic backups on the computer where you were using Zotero previously and then use Replace Online Library, as explained there.
I assume you realize this now, but setting up a new computer simply involves syncing normally, which would never do this.
Upgrade Storage
I assume you realize this now, but setting up a new computer simply involves syncing normally, which would never do this.