Ok first things first these are the possible scenarios from best to worse:
1) They spoofed your e-mail adress
2) The hacked your e-mail account
3) They hacked your computer
Now let's assume that they have actually hacked your computer. This scenario on it's own has something like 10 different sub scenarios depending on the tools used (keyloggers, Trojans, RAT, MAN attack, Brute Force, etc...). But they all require a connection at some point between the attacker and the victim. So you can actually fix this.
1) Disconnect the routers/modem's WAN cable if you have one, or if you have a dongle with 3G/4G simply take it from your home computer at put it at a computer far away from you home. (Didn't exactly understood if with dongle you meant 3G/4G or WIFI in which case you either own a router/modem (they used to be different devices but now they are sold as one) or you use a local open WIFI which is a hackers paradise.
2) A) If you have a router/modem access the device using the instructions the device came with (contact the people who sold it to you if needed) check ALL the security settings (change the device default password and user, change the password in the WIFI network and the encryption to something like WPA, WPA2 (never WEP it's really rubbish). If you can't access it at all ask how you can reset it (the is probably a button somewhere)
B) If you have a 3G/4G dongle connected directly to the computer then the problem is elsewhere since they use mobile phone's protocols and hacking them ain't easy at all (plus it is a HUGE criminal offense). Now the dongle uses WIFI to connect to your computer disable it and then perform the steps above.
C) If you have a WIFI dongle and you use a open WIFI (which could be theirs, in which case you are also performing a criminal offense, but nothing compered to hacking) it is time to chip in and get yourself a router/modem and a an internet connection unless you are prepared to learn about VPNs, encryption, etc and still live with the fear every day.
2) Now would be a good time to actually reset your computer just as a precaution (save your files before doing it). This is done because once a Trojan, keylogger or any other kind of malware has been installed, finding them is a pain in the ass even for the best Anti-Virus scanners.
Now if they have hacked your e-mail address but you still can access it just change the password and the auxiliary recovery e-mail address (just in case they changed it to theirs). Probably best to do it at a time you know the are sleeping. I doubt the are monitoring you 24/7, unless they work for the MI6, MI5 or GCHQ. Also it would be a good idea to check if e-mail forwarding has been activated (which means everything you send or receive they do too). Some celebrities found that out the hard way, because it is easy to be done and it is really devious if you think about it.
If they just spoofed your e-mail address nothing to worry about.
I hope this helped. Personally a would completely reformat my machine and burn the e-mail (change all the accounts connected to it to a new one), but i am a fan of security (actually i am kinda paranoid with security because i know the dangers).