Assess the Computer
Recognizing when you've actually been hacked is difficult, as the best attacks will go unnoticed. However, some tell-tale signs can tip you off. A situation like identify theft could indicate that your computer has been compromised, but there are other, subtler signs. Programs installed on your computer that you are sure you did not authorize is one such warning sign. Also, if your Internet connection speed is consistently slower than normal, it may be because someone is remotely connecting to your machine. Another red flag is if an empty hard drive suddenly becomes full. This is what happens when hackers hijack a computer to host illegal files or websites.
Hacking is when someone without authorization breaks into a computer system. The activities performed after a hacker breaks into a system range from storing and retrieving data without permission to damaging the normal functions of the system or network. When a person forces their way into a system they are not normally allowed access to, they are breaking the law in some fashion and should be reported.
If you suspect that you're being hacked, document everything as well as you can. Track days and times that you're noticing activity, and make sure to take screenshots of any applicable information. Also, track anything that has happened outside of your computer that leads you to suspect hacking, such as the exposure of company data or personal financial or identity theft of any kind. This is all important evidence for the authorities if they open an investigation.
Who to Contact
Typically speaking, Internet crimes like hacking are handled by the FBI. Other government organizations such as the Secret Service and the ATF also have roles to play, but hacking often falls under FBI jurisdiction. The Internet Crime Complaint Center has been established as a means for reporting cyber crime. This site acts as a central processing center where the complaints can be forwarded to the appropriate enforcement department. If you suspect that you have been hacked, you should report it to the ICCC through an online form. However, if you think your problem is time sensitive, you should contact local law enforcement for advice on how best to proceed.