|The blinking led either indicates a problem with the mboard or something connected or not connected to it, or a problem with the power supply. |
Remove the AC power to the case.
Make sure that all cards in mboard slots and the ram in it's slot(s) is/are all the way down in the slots.
Most mboards require two power connectors from the power supply - the main 20 or 24 pin one and a 4 or 8 pin one, to be plugged into sockets on the mboard. They must both be plugged in.
If the main power socket on the mboard has a 24 "pin" socket, the socket must be filled with either a one piece 24 pin connector, or a 20 pin and a 4 pin connector from the same wiring bundle, from the power supply..
If you have a video card installed in a mboard slot, if it has one or two power sockets, you must plug a connector or a pair of connectors from the same wiring bundle from the power supply into each socket, that fills the socket.
If the original power supply failed, sometimes power supplies damage something else while failing, often the mboard, and replacing the PS does not result in a working system.
That's more likely with a brand name system because they usually have an el-cheapo power supply.
E.g. if it's brand is BESTEC they are well known to be more likely to damage something else.
If you have a video card installed in a mboard slot, the power supply must have at least close to the recomended wattage capacity required for the video chipset on the card. Brand name systems often have a minimal PS capacity, and they're especially likely to not have enough capacity when it's a small form factor desktop computer.
If the capacity of the PS is more than a little under the recomended PS wattage capacity for the video chipset on the card (e.g. more than 50 watts under) , the system may not boot at all, or not fully boot, when the video card is installed, or if it does boot, the PS will be damaged in a short time because it is being constantly overloaded.
If the mboard also has onboard video, try removing the AC power to the case, removing the video card from the mboard slot, connecting your monitor to the onboard video por, restoring the AC power to the case, and booting the computer.