So, some more "basic" questions:
1. What exact Mac OS are you running?
2. If it is Mojave (OS 10.14.x), since the original version, OS 10.14, there have been 3 updates: OS 10.14.1, OS 10.14.2, and OS 10.14.3, and the most recent upgrade was a Supplemental Update to OS 10.14.3 that was released last week. I assume that is the one you mean?
3. Dropbox is definitely nowhere near enough for backing up. It is imperative that you invest in an external device, and start backing up. If you don't, you are asking for trouble.
Quite a number of folks come here for help without a backup (or backups), and thus it is difficult (and at times impossible) to provide assistance. I just hope resetting your modem/router is enough. Again, though, make sure you follow the instructions of your ISP regarding such a reset. Each ISP is different, and it is (in most instances) not something as simple and quick as shutting down, then starting up, the modem/router. If you don't follow the instructions, it could make matters worse!
One other thing: whenever you want to make an update (and especially with the Mac OS), it is so, so critical to have a backup first.