If they had you reset it, but chose the option to try and keep your personal stuff on the device when you reset it, that is likely why it is giving you trouble. That kind of reset really doesn't fix anything if it is a software problem. The only way to really do that is to do a "Hard Reset" which wipes everything and then starts the device as it was originally, prior to your usage.
You can try turning it off and back on and see if it will start. If not, then try starting it up in "Safe Mode" and see if it will load. If it does, then you need to save all your stuff from it either to a google account, computer, etc., so as to not loose it. All files, contacts, etc., except apps, you can't save those, then do the "Hard Reset". If the problem is the system, then it should be fixed once it finishes the reset. However, if the problem is hardware, then it will need to be repaired or replaced.
Also, on a side note, for warranty issues (if the device is still under warranty) you should contact the manufacturer, not the carrier/service provider. Unless you are still paying them for the device that is.
To get into "Safe Mode" please do the following.
1. First turn off the phone.
2. Next press and hold "Power" until you see the manufacturers logo on the screen, then release the power button.
3. "Quickly" press "Volume Down" and hold it for few seconds until you see the message "Safe mode: ON" or something similar.
4. If you wanted disable it, simply repeat the steps above, and you should see "Safe Mode: OFF" rather than on.
NOTE: Doing this will clear 'all' the data on your phone, so it is suggested you not choose this option unless you have to, or aren't worried about losing anything on the device.
Be sure you back up any data you want to keep, to a computer or cloud account, before doing the reset.
To "Hard Reset" the phone...
1. Make sure that the phone is actually powered off.
2. Remove your SD card, if there is one in the device.
3. Press the "Power" and the "Volume Up" buttons, together, until you see the start-up logo on the screen.
4. Once you can see the logo, release the buttons and allow the device to enter the "System Recovery Mode".
5. Next use "Volume Down" button to get to the "wipe data/factory reset" option.
6. When in recovery mode the "Volume Up" and "Volume Down" buttons are how to can navigate and highlight what option you need.
7. Once you have selected the "wipe data/factory reset", press the "Power" button to select that option.
8. On the following screen, repeatedly press the "Volume Down" button until the "Yes - delete all user data" option is what is selected.
9. Then press the "Power" button to hard reset the phone.
10. If you took your SD card out in step 2, when the device is up and running again, reinsert the SD card.