Sometimes this virus (which has numerous different forms but always masquerades as an anti-virus/security program) will prevent you from running Malwarebytes and possibly even stop you from accessing the internet (dead giveaways that you have a virus, btw).
I've had this happen to me and the solution is to get on the internet (with another computer if need be; or run in safe mode and see if that works) and Google the name of the security program/virus. Be careful which website you go to because sometimes the pricks even make fake websites. If it's a link to a website that wants money, it's bullshit. Your best bet is to look for Google links to websites that are computer forums. There you will find multitudes of other people who have had the same thing and usually a step-by-step instructions for how to disable and remove the virus.
Sometimes you have to download and run a little program that will stop the virus from running, thereby enabling you to run Malwarebytes to detect and remove the virus.
Once you've been through it once, you'll know what to do the next time and it's more a pain in the butt than a real threat to your computer.