For the study of medieval Latin hymns, sequences, tropes, and the like, the following mighty but clumsy resource has yet to be superseded:

Analecta hymnica medii aevi, ed. Guido Maria Dreves, Clemens Blume, and Henry Marriott Bannister, 55 vols. (Leipzig: Reisland, 1886–1922).

Scans of all the volumes are freely available at the Internet Archive:

AH 01	AH 12	AH 23	AH 34	AH 45a  45b
AH 02	AH 13	AH 24	AH 35	AH 46
AH 03	AH 14a (no part b)	AH 25	AH 36	AH 47
AH 04	AH 15	AH 26	AH 37	AH 48
AH 05	AH 16	AH 27	AH 38	AH 49
AH 06	AH 17	AH 28	AH 39	AH 50
AH 07	AH 18	AH 29	AH 40	AH 51
AH 08	AH 19	AH 30	AH 41a (no part b)	AH 52
AH 09	AH 20	AH 31	AH 42	AH 53
AH 10	AH 21	AH 32	AH 43	AH 54
AH 11	AH 22	AH 33	AH 44	AH 55

(It was only after I had laboriously assembled this collection of links that I discovered a page where someone else had already done the work.)

The series was made more useful by the publication of several volumes of indexes (not available online):

Analecta hymnica medii aevi: Register, ed. Max Lütolf with Dorothea Baumann, 2 vols. in 3 (Bern: Francke, 1978).

There also exists, however, a complete online database of the whole of AH:

Analecta hymnica medii aevi digitalia (Augsburg: Dr. Erwin Rauner Verlag, 2002–2023).

Full access requires a paid subscription, but the publisher provides a page that allows free simple searches of the index, and this makes it possible at least to find out which volume contains any given hymn.