Engine ran great, monkeyed with TCM now won’t start

[Hope]

Help. I know just enough about cars to get myself in trouble. Engine itself ran awesome. My problem started with my 2000 300M going into limp mode. Codes p0700,0750, p1768,p0731. Changed speed sensor . After a few miles went back to limp mode. Checked relays, before having someone get into my tranny (have no money) I changed the TCM (used) . Car Barely ran after that. So changed back to old TCM. Turned over, ran a few seconds. Then nothing not even a click. Please Help a poor girl. My only car and stuck 29 miles from town

[FIREM]

1768 Power and ground Issues. If the engine wont even crank/start check all Battery, jump post and ground post connections by removing, cleaning and tightening them. Also check the connectors for the trans near/under the brake booster/master. 2 Grey connectors and harnesses that are near the tie rods.
731 Usually Speed sensors (2) did you change both? OEM ?

[Hope]

Did playing with the TCM cause any of the problem or is it just a coincidence that after my engine won’t engage

[Hope]

Changed the speed sensor but not the other one

[LUNAT1C]

Change both speed sensors. There is an input and an output. OEM Mopar only for any sensors on this car, aftermarket doesn't work.

I second checking all power and ground connections. Both at the battery, both jump points, engine grounds (one on each side of the engine, straps connecting the engine to the subframe), power distribution center battery connection (I had forgotten to tighten that one myself when heading out on a road trip and thought I had a dead car in my driveway...).

[FIREM]

TCM near the PCM may have disturbed some of the connections. Double check that all connectors are properly seated in both TCM & PCM

[Hope]

Thank u so much. I will do that and report back if it might help someone else. Someone told me I have to have it flashed before it will run again. Is that true?
Again I am so thankful for your help. I have been at my wits end. I was told to have a guy look at it. But sometimes it isn’t an option and I know my way around for the most part