If you can get access to a parallel programmer then you can read the boot vector and status byte and see what they are. The Boot Vector should be FCH. The status byte would likely be 00H = execute user code. Programming the status byte to FFH will ensure on the next power up that the device will enter ISP mode.
Even if the parallel programmer for some reason did not offer the option to change the boot vector, you can at still reprogram the device with a simple program that uses IAP routines to reprogram the boot vector to FCH. See http://www.esacademy.com/faq/progs/flash.htm for a library that allows you to use IAP from C. You would have to call iap_init and iap_erase_boot_vector_status_byte() followed by iap_program_boot_vector() and iap_program_status_byte() - that should be all you need.

If you are not sure if you are placing the device in ISP mode correctly, make sure you are also pulling P2.6 high as well as P2.7 high during reset and that your voltages *on the device's pins* are what you expect them to be.

Andy
Embedded Systems Academy