Usually if there is no signal to the monitor, the computer also emits several beeps. Very common and relate to memory error, especially after delivery if badly handled or desktop been moved from one place to another or case hit/punched. To solve we will suggest to open cover and remove all memory RAM sticks and clip it back again. This should solve your issue.
How to - YouTube video:
Dell 755-780 SFF: https://www.youtube.com/watch?v=uNkMR6_Tc5s
Dell 755-780 DT: https://www.youtube.com/watch?v=bXnxgMduQs4
Dell 755-780 MT: https://www.youtube.com/watch?v=0r_-2MdJhsk
Dell 790-9010 SFF: https://www.youtube.com/watch?v=87tjRkRoefY
ThinkCentre M58 SFF: https://www.youtube.com/watch?v=GzNqRxQMSDk
ThinkCentre M91p SFF: https://www.youtube.com/watch?v=dnmGy0guUvM