Short answer: nothing dramatic. Here is exactly what the fund house does, what your bank might charge, and when a SIP actually gets cancelled.
The fund house does nothing
If your bank account does not have the money on the SIP date, the instalment is simply skipped. The AMC does not charge a penalty, and your existing units are untouched.
Your bank might charge
The debit is attempted through your e-NACH mandate. Most banks charge a fee for a failed auto-debit — typically ₹100–₹500 depending on the bank. That is the only real cost of a missed SIP, and it is the bank's, not the fund's.
When a SIP gets cancelled
Most AMCs cancel the SIP automatically after three consecutive failed instalments. You can restart it any time with a fresh registration; your old units stay where they are.
If you know money will be tight
Pause, don't miss. Almost every AMC lets you pause a SIP for one to six months with no charge. Ask your associate or do it on the AMC's website a few days before the date.
The part that matters
One missed month changes very little. Stopping altogether after a bad month changes a lot. If you have to skip, skip — and start again next month.