It depends on what you’ve done: 

• Just opened it? That’s OK (you’re unlikely to have realised it was dodgy otherwise). Just delete it so you don’t accidentally do worse. 
• Clicked on a website link/opened an attachment inside? You’ll probably get targeted for SPAM but if your security updates and anti virus software are up to date you should be OK, although the latter will likely be busy. If your device displays unusual behaviour though, report it to IT ServiceDesk immediately. 
• Entered your username & password somewhere? Noooo, that’s the bad one – someone else now almost certainly has control of your account. Try and change your password / contact the IT ServiceDesk immediately on 0800 nmitIT.