100 tax payments through ITC

One of taxpayer received notice from state dept regarding non payment of tax liability from cash n utilizing itc for 100% outward liability.
Turnover is below 2cr , all supplies are b2c.

Notice doesn't contain DIN and no
any section references is mentioned in notice. provided taxpayer to produce purchase, sales invoices transportation details etc within 30 days.

my questions are

1. is there any restrictions for such itc utilization for small taxpayer.
2. validity of notice.
3. consequences for not responding to such notice if notice is invalid.