Do not worry my friend. Though you may have heard that big companies mostly recruit from campus and if you are not selected in campus interviews then it is tough to find job in big companies from outside, still this is not a rule. Apart from campus some big companies do recruit from outside also through written test followed by interview. Most of the PSU and government companies like BHEL, BEL, NTPC, DRDO, ISRO etc follow this approach so you need to be alert and look out for their advetisements in newspapers and emplyment news. But competition here will be tough and you should be well prepated with all your BTech subjects to clear the written test. I think since you are just passed out everything must be fresh in your mind, so do not let it fade away and keep brushing your knowledge in meantime. As far as private big companies are concerened some of them do hire from outside by calling the suitable candidate for direct interviews though sometimes small written test is also there. So again here you need to keep open your eyes and ears and look for the relevant openings in different sources when they come out.
Though in earlier days it was said that it was easier to find job when experienced rather than fresher now the trend has changed. Nowadays due to cost cutting adopted by companies they prefer to hire the freshers more and train them rather than hiring experienced people and pay them more. So currently I feel recruitment for freshers is picking up so you do not need to worry. Yes, but in in your search if you get selected with small companies please do take it up and then continue your search with bigger companies. Right now try for all types of companies rather than only big companies.