Offer letter and appointment letter is
two different things. Offer letter is whats they have offer to you whereas appointment letter is agreement of service agreed by both employer and employee accepting the terms thereof.
Keep the documention ready pertains to your employment if any?
With regards to Non Payment of salary :- since your salary is 17K Hence you are covered under payment of wages act. 1936. As sec 4 of the said act. No wages pariod can be fixed more then 1 Month which should be disbursed within 7th days or before after closing month if the employees strength of origanization is 1000 or below however if the strength is more then 1000 then wages should be disbursed within 10th or before after closing month.
Send written application through registered post seeking your dues if they remain adamant then you can initiate legal action again them. Also you can file the case u/s 33C(2) of ID act. 1947 to recover your dues if you are categorized as workman?
Thanks & Regards
HR & Labour, Corporate Laws Advisor