ATmega328P firmware flashing interrupted - is it fixable?
It seems likely that you have inadvertently changes the fuses to one of:


Disable /RESET
Different clock speed or clock source
Disable SPI programming


To recover this you need to use a "high-voltage" programmer that involves holding /RESET at +12V whilst specific sequences of commands are sent through most of the other pins. This may be difficult with your "pogo pin" arrangement.

It may even be impossible if putting 12V into /RESET damages other circuitry (eg. if you have a pull-up resistor that would bleed 12V into the 5V line).

I have a post about using another Arduino to do high-voltage programming here.



You can see that there are a lot of wires, and it may not be practical to do what with pogo pins.

You can also use an AVR Dragon board to provide the necessary signals and pin-outs.



However this would require that you either remove the chip and place it inside the programmer, or run something like 18 wires to the board, similar to the earlier solution.



I suggest, whilst testing, that you solder on a 6-pin header suitable for doing your SPI programming. That will eliminate the pogo pins, and give you a reliable contact for programming. Once you have debugged it the production models won't need that.




  What about external oscillator?


You could use my chip detector sketch which would help show if the chip is unresponsive, or has merely had some fuses changed (eg. clock source). 

Example output from the sketch:

Atmega chip detector.
Entered programming mode OK.
Signature = 1E 95 0F 
Processor = ATmega328P
Flash memory size = 32768
LFuse = FF 
HFuse = DE 
EFuse = FD 
Lock byte = CF 
Bootloader in use: Yes
EEPROM preserved through erase: No
Watchdog timer always on: No
Bootloader is 512 bytes starting at 7E00

Bootloader:

7E00: 11 24 84 B7 14 BE 81 FF E6 D0 85 E0 80 93 81 00 
7E10: 82 E0 80 93 C0 00 88 E1 80 93 C1 00 86 E0 80 93 
...
7FE0: E7 DF 80 32 09 F0 F7 DF 84 E1 DA CF 1F 93 18 2F 
7FF0: DF DF 11 50 E9 F7 F4 DF 1F 91 08 95 FF FF FF FF 

MD5 sum of bootloader = 0F 02 31 72 95 C8 F7 FD 1B B7 07 17 85 A5 66 87 

First 256 bytes of program memory:

0: 0C 94 35 00 0C 94 5D 00 0C 94 5D 00 0C 94 5D 00 
10: 0C 94 5D 00 0C 94 5D 00 0C 94 5D 00 0C 94 5D 00 
20: 0C 94 5D 00 0C 94 5D 00 0C 94 5D 00 0C 94 5D 00 
30: 0C 94 5D 00 0C 94 5D 00 0C 94 5D 00 0C 94 5D 00 
40: 0C 94 80 03 0C 94 5D 00 0C 94 C9 00 0C 94 5D 00 
50: 0C 94 5D 00 0C 94 5D 00 0C 94 5D 00 0C 94 5D 00 
60: 0C 94 5D 00 0C 94 5D 00 E5 01 11 24 1F BE CF EF 
70: D8 E0 DE BF CD BF 11 E0 A0 E0 B1 E0 E0 E9 F8 E0 
80: 02 C0 05 90 0D 92 A2 32 B1 07 D9 F7 11 E0 A2 E2 
90: B1 E0 01 C0 1D 92 A2 3C B1 07 E1 F7 10 E0 CA E6 
A0: D0 E0 04 C0 22 97 FE 01 0E 94 42 04 C8 36 D1 07 
B0: C9 F7 0E 94 1F 02 0C 94 46 04 0C 94 00 00 08 95 
C0: FF 92 0F 93 1F 93 06 EA 11 E0 C8 01 40 E0 52 EC 
D0: 61 E0 70 E0 0E 94 FA 00 C8 01 0E 94 44 03 C8 01 
E0: 60 E0 71 E0 0E 94 75 03 91 E2 F9 2E E0 E0 F0 E0 
F0: F0 92 57 00 E4 91 C8 01 6E 2F 40 E1 50 E0 0E 94 


Source for sketch on GitHub - in "Atmega_Board_Detector" folder.