What happened to Kripa after Mahabharata War?
Kripacharya was blessed with longetivity till the end of the Kalpa (Chiranjeevi) and also is scheduled to become one of the Saptarishis of the next Manvantara, as I discuss in this answer.
Also, after the Mahabharata war, as discussed in this answer:
Kripa was installed as the preceptor and Parikshit was made over to him as his disciple.
But, after the decline of the Kuru dynasty, at the advent of Kaliyuga, he must have resorted to his Ashrama, as evident from this verse from the Śrīmad-Bhāgavatam Purāṇa:
गालवो दीप्तिमान्रामो द्रोणपुत्र: कृपस्तथा ।
ऋष्यशृङ्ग: पितास्माकं भगवान्बादरायण: ॥ १५ ॥
इमे सप्तर्षयस्तत्र भविष्यन्ति स्वयोगत: ।
इदानीमासते राजन् स्वे स्व आश्रममण्डले ॥ १६ ॥
O King, during the eighth manvantara, the great personalities Gālava, Dīptimān, Paraśurāma, Aśvatthāmā, Kṛpācārya, Ṛṣyaśṛṅga and our father, Vyāsadeva, the incarnation of Nārāyaṇa, will be the seven sages. For the present, they are all residing in their respective āśramas.
Thus, as per Śrīmad-Bhāgavatam, currently, Kripacharya is residing in his Ashrama (but I don't know where the ashrama might be) and will become a Saptarishi in the next Manvantara.
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