All Jetpacks come with a restore default button under the back cover. Power up the device, remove the back cover and press the reset button with a paper clip for 5-10 seconds. When the device reboots it will reset the password back to the default provided on the sticker.
If the default password does not allow you to connect then you need to clean up the wireless profiles on the device that you are connecting from. There should be some kind of wireless profile cleanup utility or procedure if you check out that devices support forum.
Once you have a connection again make sure you do not stay on the default Jetpack password for too long. It is a best practice to change the default WiFi password on all Jetpacks to something else. I currently suggest using the MS strong password criteria for new WiFi passwords, but you are certainly welcome to go beyond that.
You can also determine the quality of your WiFi password with a site like this:
I see, that's the only one I am now aware of that does not have a back cover. Makes me wonder how the battery is ever swapped out. Closer examination reveals they do not want you to be able to with this model: "Your Jetpack is equipped with an internal, non-removable, rechargeable battery. It will work from its charged battery alone, or when the device is plugged into a power source." Bummer.
In that case then you would likely have to contact VZW and find out the proper reset procedure when the admin page cannot be accessed. I searched through the User Guide and was unable to find anything identifying a reset button or reset procedure. There has to be some way to accomplish it otherwise VZW is going to have to send you a new one.
You can see wifi password easily by pressing the power button shortly a couple of times.
First press will wake up the display, second press will bring data usage meter, third press shows wifi name, then fourth press displays password. Try that password.
Another tip about changing wifi name and password is..when you change the both wifi name and password together, it's OK becaue your pc will identify new wifi network and ask for password. The problem is when you change the wifi password only and keep the wifi name as it was. Because your PC has already wifi network profile saved (wifi name + password), when your PC see the same wifi name, it will keep trying to connect using old(saved) password. In this case, you can simply choose erase (or forget) the password. The way to do it vary depeding on your PC and OS. In case of Window 8.1 (mine), when you try to connect the same wifi name after you changed the password, it shows "Can't connect to this network", then show buttons Forget network and Close. Click Forget network. You can still see the wifi name on the list. Then click connect. It will ask for password. Now you enter new password. that's it.