You can fix the paths in batch with the Zutilo plugin.

The way to avoid it going forward is to make sure you've set the Linked Attachment Base Directory in the Advanced → Files and Folders pane of the Zotero preferences. That will cause Zotero to store all linked file paths relative to the path you set, so that if you, say, move to another computer where the path is different, you simply need to update that setting and the relative paths will still work.

Just found this. Thank you.

Zutilo is amazing!

@dstillman

@qqbb, there are hundreds of posts of you guys talking about ZotFile and Zutilo before my question came in. If you do not want to help this time for some reasons it is fine, but do not tell me that I should not be asking the question because similar questions have been asked millions of times.

Thank you for the clarification. This is my point of view, though, which anyone is free to criticize: his link is useless because I still do not understand how to solve my problem, because the information from that link is like Chinese to me. Thanks for the link, though.

@OronzoAdrianoCelentano My comment was not meant to be offensive, sorry if it seemed so. I'm a Zotero user just like you. Occasionally, I try to help. Your questions can be helpful for others as well, I'm sure many users will face similar problems. Zotero and its plugins are great software, but many functions are indeed quite involved and often it's not obvious how to do something. Luckily, this forum is a great resource. I hope you'll get the help you need.

Regarding Zutilo, did the installation work for you? Zutilo can add further options to the context menu of an item, which is accessed by right-clicking. You need to make them visible through "Tools" -> "Zutilo Preferences..." -> "User Interface", e.g. by selecting "Zutilo context menu". (Selecting "Zotero context menu" just changes the level in the item context menu, which could be useful for frequently used functions.) For the purposes here, select "Show attachments" and "Modify attachments", which add the "Show attachment paths" and "Modify attachment paths" functions to the context menu.

The "Modify attachment paths" function can change the file links (pointers) in Zotero's database. Note that it will not move the actual files from one path to another. The function prompts for the old and new partial paths. In order to figure out the paths of broken or working links, you can use the "Show attachment paths" function. This could show you an absolute path of a linked file ("C:/userData/references/coolPaper.pdf"), a relative path of a linked file in the Linked Attachment Base Directory ("attachments:references/coolPaper.pdf"), or a path to a file in Zotero's storage ("storage:coolPaper.pdf").

I would like to encourage you to describe in detail what your problem is. For properly fixing it, without creating possible additional issues from failed attempts (remember to create backups), it would help to know how the problem occured (e.g. from changing a computer or moving a folder) and what the old and new file paths look like (e.g., whether you're using relative or absolute paths). The more details you can provide, the easier it will be for others to help. As dstillman suggested, it might be useful to open a new thread, providing as much information as possible.