Get Android ICS stock firmware for Lenovo S720 Smartphone from here or from here. Get previous firmware from here.
Firmware details
Archives: Lenovo_S720 4.0.4 fw
Image: Lenovo_S720_VIBE_ROM_SJDS_20150315 / Lenovo_S720_VIBE_ROM_SJJL_20150315
Archives: Lenovo_S720 4.0.4 fw
Image: Lenovo_S720_VIBE_ROM_SJDS_20150315 / Lenovo_S720_VIBE_ROM_SJJL_20150315
Model: Lenovo S720 Smartphone
Image file size: 471 MB
Build number: S720-user 4.0.4 IMM76D S720_S137_130827 release-keys
Caution: Please do not update firmware unless you know what you are doing, you really want it or it is absolutely necessary. Firmware updates may occasionally have bad results, like bricking your device, temporarily or permanently. The firmware update process is done entirely at your own risk. Before attempting to update the firmware, back up your data