This is very common and is called browser hijacking. Why has the start-up browser page changed and how can I fix it? A malicious program—or at least an unwanted one—has corrupted your computer. Another example: While installing part of the software, your browser’s options change without your permission. The way it changes varies, but the end result is usually the same: more advertisements.
You have changed the start page or have toolbars that you have not installed voluntarily.
The “kidnapper” is earning money thanks to this practice (which is sometimes harmless but always very bothersome). Depending on the malware that is installed on your machine, it can be more or less difficult to get rid of. Don’t forget to take precautions.
In this case, there is a measure that almost never fails: installing antivirus software and keeping it updated. Good security solutions are able to recognize the malicious programs and prevent them from getting into your computer.
What if it’s too late?
If it is too late and your browser has been kidnapped (ex. it runs slower than usual, displays advertising for unusual sites, you see toolbars that you didn’t install, the home page has changed or your searches are redirected), then you have no other option than to restore the configuration to get everything back to normal.
For the most popular browsers, including Google Chrome, Mozilla Firefox or Internet Explorer, you can restore the configuration using the simple instructions online.