Latin had six noun cases, but in all Romance languages except Romanian, nouns only have one singular form and one plural form left.
What happened?
This extra large infographic tells you in ten steps how the Latin case system collapsed – and how the accusative case emerged victorious:
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What caused the case system to collapse in the first place?
There were multiple factors.
Regular sound changes did a lot of damage: many endings had come to be pronounced the same:
-ō and -um;
-is and -ēs;
-us and -ōs;
-a, -am and -ā;
-e and -em.
This obfuscated the system a lot.