Lost water pressure in my whole house after installation of a dishwasher??

ok here's the deal, I have no clue what happened, my girlfriends dad and I installed a dishwaher, onto an existing line under my sink, the pipe cement he was trying to use and let set up, kept blowing off, and we would have to do it all over again, this was on standard PVC, straight from the dishwasher to the line under the sink, after this happened so many times, i became frustrated, we turned the water off for the night, essentially the water pressure was fine until this point, so i found the lines that led to the sink and kept them off all night, we seemed to have lost pressure already, the next morning I went out and bought a Sharkbite T and took out and replaced the crap job he did, and then turned the water back on completely again, still had horrible water pressure, i've looked to see if there are leaks, there are none, i've tried re-pressurizing numerous times, i.e. pulling all the knobs out on the sinks and tub letting the air to come out, then turning the water back on, from top floor to bottom floor, and vice versa.. none of this seems to work, we still have no pressure, and it all started when we installed the dang thing, any suggestions.. i did find a little sediment in my sinks and the back of my toilet, i'm hooked to city water, and i do not have a PRV in my lines anywhere, it comes from the street, to the water mainline in the basement, to the main on/off valve, and then to the water reader, where there is another valve that needs open for the water to be full, so if you have any idea whatsoever, it would be great, like i said no leaks, just no pressure in the house, well just enough to shower, and flush the toilet, which i might add takes forever to refill.. Please suggestions..

My guess is the main shutoff valve is shot and won't open full bore. You may have to shut the water off at the curb and replace it. I suggest a full bore ballvalve.

When sediment in the lines got churned up, they could have plugged the aerators and pressure balance spools but then the pressure would still be OK where there are no aerators and/or pressure balance spools.

CPVC can be used for your supply lines, but PVC CANNOT!

As janashua notes, PVC should not be used inside. However, that alone is not your immediate problem. You made not reference as to how you transitioned from the DW to PVC and to the water supply. I suspect you used PVC cement which not only will not join PVC to other media, but used it to excess and that has clogged you pipe. One of the best and easiest ways to connect a DW to the water supply is with a flexible hose with stainless steel braid. The connections are all done with fittings, not cement. It is very possible, even likely that you have PVC cement inside the DW valve and this will have to be dealt with. Most likely with a new valve. Even using CPVC, you would have to have transition fittings and without the flexibility of a hose, the connection would be very difficult.

I one time turned off a gate valve that stripped and wouldn't allow the stopper to pull back up.
The entire home is low pressure now?

How hard was it to turn off the valve - I aggree - you broke the stem of the valve

Maybe post a picture of your main shut off valve ? I agree with the others, sometimes the valve feels open but the disk hasent opened all the way. Is the handle just turning and turning and not stopping? this is also another indication the gate valve is broken.