A couple of things:
Resetting to factory defaults is the simplest way to do this, and is what you're going to want to do. However, before doing that:
There are many methods a router can use to connect to the modem and get service from the ISP. The risk you run by resetting it to defaults is that if the default settings are not appropriate, then you won't have internet access until you set it up again (it is not uncommon, however, for the default settings to be OK). These settings are not particularly complex, but if you have no experience setting them up and are uncomfortable poking around through settings, then you may have trouble getting it to work on your own. That said, your ISP will give you instructions for these settings, generally without hassle, if you call them. Note that if it works without you having to set anything up after resetting, then it is officially working - you won't need to "make sure" or anything like that; it will either work or it won't, and it's not going to be permanently broken if it doesn't.
If you do reset it, internet access aside, you'll want to go in and set up your wifi settings again too (e.g. network name and password). It's generally straightforward, but if you're intimidated by this type of configuration, you might not feel comfortable doing it. Just do it; refer to the user manual if necessary. In the worst case you just reset it to defaults again and start over. You won't hurt anything.
Whether it's your own router or not, you're not going to get in "serious trouble" for resetting it. Best case is it works immediately. Worst case is you have to reapply the settings your ISP gives you.
What I would recommend for you is the following:
- Try 743's solution if you're using the Cisco Connect software. It's an easy thing to try and worth a shot. Failing that:
- Call your ISP's customer service and explain that you want to reset your router because you forgot the password. Ask them what settings you'll need to connect to the modem (they may give you connection method, IP address settings, etc.). Note that if you still have the receipt laying around from when the ISP initially installed everything, often times the tech will scribble this on the receipt (or it will be printed there), saving you a call to the ISP.
- Write these down and keep them somewhere safe; if you ever need them again you won't have to call the ISP again.
- Reset the router to factory settings. Connect your computer to it with an ethernet cable, or with wifi if wifi is enabled by default (consult user manual for default wifi network name and password).
- Log into the router using whatever method; usually either through a web browser or through some custom configuration utility (consult user manual).
- Set up internet connection as per ISP's settings. Set up wifi to taste.
- Tada. And don't forget your password this time.
People's routers get reset for various reasons, and people lose their router passwords a lot as well. This won't be a weird call to the ISP. It's a fairly common and reasonable request for information.
Amusingly, my ISP here was apparently bombarded with so many calls from people losing their router / wifi passwords that they now give all users a modified Netgear router with their service that has custom firmware on it to let them access it remotely and reset your passwords.