New RAM in Pre-built PC - Won't Boot

If your PC is the H320, then the problem is the RAMs because the PC can support up to total 8GB RAM, that means you can only use the max. 2GB per stick( 2X 2GB) not the 4GB module.
Here is the h320 full specifications. http://www.lenovo.com/shop/americas/content/pdf/desktops/Essential/h-series/Lenovo_H320.pdf

My compliments to CIN 19 on providing info on the "compact" Lenovo PC and a URL citation for further mobo specs. I only use this machine as a "back-up" because this particular model uses a non standard [size wise] power supply and a paltry 180 watts max power with exactly TWO connections coming out of the non standard size PS, one for the hard drive and the other for the DVD/CD player. That's it! There is not a single wire extra to even piggy back [or substitute an updated plug, example, extra 4 or 6 or 8 pin DC power for a upgrade to a decent external video card versus the on-board video chip the computer comes with. Even if it did have extra PS wires for today's upscale video cards which 'require' additional DC power, with 180 watts, it's far too low to run any of the more powerful watt hungry video cards. Strange .. when I bought it brand new some years ago it had an Anatel router connection gizmo built in [!] [taking up about an inch and half slot on the mobo and, other than that mini slot, there is only one other normal size PCI-E slot for an external video card] along with a dual wired external antenna for the two male receptacles and it connected to my home network [now] Belkin 750 router without any problem once I typed in the router security code. I didn't care for this "compact" model with its non standard size PS and to get to the HD [I use a 1 TB SATA drive on it now] you have to do some 'surgery' to get to it as the internal DVD/CD is in the way [and which operates vertically in this "compact" computer] and sits on top of the HD with its 3 screws and push to release housing. Give me a roomy tower system anytime! As CIN 19 stated, it can handle 8 GB's of memory but you MUST use 2 GB sticks! The machine will reject a 4 GB stick and will fail to start if it doesn't have 2 GB memory sticks [1333, DDR3]. The built in CPU fan is barely adequate to do the job of cooling the CPU [in my machine, a 3.20 gig CPU] and, get this, often the CPU fan will not even begin to turn until the O/S [I use Win 7, 64 bit] starts! As I said, I keep it around as a "back-up" machine and for extended word processing. --- DocTony