This is the BIOS telling you that something's wrong. You need to check all external connections 1st, if problem persists then check all internal - including seatings.
- Failing that determine your motherboard's make and model then Google these + "manual".
Since not all motherboards produce the same BIOS beep warnings you need to...
- Download manual, check out troubleshooting & pinpoint issue.
- Resolve issue.
Many such issues are very simple to fix e.g. keyboard/mouse not detected, memory not seated correctly, etc. If a device/card (e.g. gfx) is suspect, you may want to try it out in someone elses PC before replacing with a new one.
As swiftly mentioned above, have you installed any new hardware into the PC - e.g. gfx card? if yes, make certain this (and any other new device) is getting enough power...ultimately, your PSU needs to be fit for purpose within your system.