My lawyer sent a legal notice on September 23rd regarding a bounced cheque. Is there a deadline for filing a case under Section 138 of the Negotiable Instruments Act? Can I still file a case against the other party?

Yes, you have 30 days from the date the cheque was returned unpaid to file a case under Section 138 of the Negotiable Instruments Act. As your lawyer sent the legal notice on September 23rd, you can still file a case as long as the cheque was returned unpaid within the past 30 days.