A theft in which any individual or organization hijacks and changes the registration of the domain name improperly is called domain name hijacking or theft. It is done without the knowledge of the original owner and it can be used for many malicious reasons or to get notoriety. It can also be used to sell the domain again to the owner for a large amount or any third party in order to increase business.
If your domain is hijacked then you must first contact the company who provided you the domain. The registrar will then try to get the domain back to your name which in rare cases is less damaging. It is not necessary that you will get all the data back and the third party hasn't made any changes to harm your online business and reputation. However, sometimes it is just easier and cheaper to buy a new domain and start over again then to do the transfer process and pay a hefty fee.
It is highly advisable that before you buy a domain, you should know that the registrar who is providing you with that domain is reputable. There are many providers out there so we need to choose carefully. It is also advisable to get all the domains from the same registrar so that they can be monitored easily. You also need to make sure that you change your contact information along with your username and passwords frequently. Registrar send you information on emails and whoever can access your email can also access your domain. So you need to monitor all of these things and use tools such as registrar lock etc.