For hardware problems, start by checking the computer's power supply; for software issues, scanning for and removing viruses can fix a number of issues. If the computer's files are backed up, reinstalling the operating system sometimes help.
If the computer doesn't start, check the power cord to make sure it's connected to both the computer and the outlet. If it still doesn't turn on, test the outlet by plugging in another device to make sure it's receiving power. The problem might also be with the monitor; if the computer's lights turn on and its fans run, check both the monitor's power cable and the cable attaching it to the computer.
If the computer runs poorly, the problem might be related to a virus or other malware. Windows 10 comes with a virus scanner in its Windows Defender suite, and there are a number of free antivirus suites available. Windows 8 and later editions let users "refresh" Windows, which reinstalls a number of components and sets some settings to default values. This refresh can fix some issues. If this doesn't work, reinstalling the operating system might fix the problem. If it doesn't, the problem might lie with a faulty hardware component, such as the RAM or the disk drive.