I put the sentry key in to start the truck but it shut off right away and i had a flashing security light on my dash. now it wont start at all. the security light stays lit than flashes when i try and start it. what should i do ?

 

From the owners manual:

SENTRY KEY IMMOBILIZER — IF EQUIPPED
With this system, an electronically coded ignition key
sends a signal to the vehicle electronics. If the electronics
recognizes the signal, the vehicle will start and continue
to run. If the system does not recognize the signal, the
vehicle will start and run for 2 seconds, then shut off.
After six unsuccessful attempts at starting, the system
will shut down until the correct key is used.

That's exactly what happened when I cut a new key and tried to start the truck without programming the key. So it looks like your truck is not recognizing the key.
If you have another key, try that one.

Or, try and disconnect the battery for a while, that clears up a lot of electrical problems.

Welcome to DF, we'll help you out the best we can here.

 

Some more info:

The Sentry Key Immobilizer System is not
compatible with remote starting systems. Use of these
systems may result in vehicle starting problems and a
loss of security protection. Additional Sentry Keys or
Mobil Speed-pass™ devices held against or immediately
adjacent to the ignition key when starting the engine may
cause vehicle starting problems. If a problem occurs,
remove the Sentry Key from the key-ring and attempt to
start the vehicle again. Pagers, cell phones, walkman, etc.
will have no effect on this system.

The Security Light will illuminate for about 3 seconds
when the ignition switch is first turned to the ON
position. If the vehicle electronics do not receive a valid
signal from the ignition key, the Security Light will flash
continuously to signal that the vehicle has been immobilized.

If the Security Light remains on during vehicle
operation, it indicates a fault in the system electronics. If
this option was ordered, all of the keys provided with
your new vehicle have been programmed to the vehicle
electronics.

 

When my battery died on my truck... LIKE DEAD DEAD needed to be replaced that red light was flashing on my truck.

But I do not have a sentry key... just throwing out other options.

 

The truck will start with out the sentry key using a plain jane key.
The battery died the other night from the cold less than a year old.
I charged it the truck starts fine but now the red security light flashes and so does the head lights and maker light along with the tail lights.
These keep flashing even when you switch the truck off, but you can unhook the battery reset it. It quits but once you open the door the security light comes back on along with the head lamps, the truck starts fine but the lights keep on flashing. ANY HELP?

 

did you truck come with 2 keys? mine did, I had to start using the other key to solve all my issues, something to do with the wear of the key from using it so much, the chip and computer weren't communicating together anymore

 

If it's not a key issue and the battery is good, clean and tighten up the positive lead from the battery to the fuse box. This may be the issue if doing it intermittently.